Liquid crystal display has been used in various electronic devices. Because liquid crystal molecules do not generate light by themselves, a back light module is required for providing light on a liquid crystal display.
Typically, there are two types of backlight module in liquid crystal display. One is always-lighting type backlight module. The other one is flash type backlight module. The flash type backlight module, for example, is applied to liquid crystal display driven by a color sequential method.
According to the color sequential method, three primary colors are sequentially switched in the persistence of vision time to compose a color. That is the primary colors are sequentially displayed in three time segments. Therefore, a complete color image is displayed as a rapidly changing sequence of primary monochrome images. Since every pixel unit in the display contributes to every primary image, a color sequential imaging display must address pixel unit first to select which pixel unit to display.
When addressing the pixel units in a liquid crystal panel, the scan lines are scanned from the top to the bottom. Then, the backlight module is turned on to display a specific gray-level image on the panel. Since the top scan line is scanned first and the bottom scan line is scanned last. Therefore, a time difference exists in these scan lines. Such time difference causes different response time for liquid crystal molecule to rotate. That is the liquid crystal molecule located in the top scan lines has more response time to rotate. The liquid crystal molecule located in the bottom scan lines has less response time to rotate. Therefore, an over driving technique is disclosed to compensate the response time difference. Accordingly, a specific voltage is corresponded to a specific rotation angle of liquid crystal molecule. A response time is required for liquid crystal molecule to rotate to the specific angle. When a higher voltage is applied to liquid crystal molecule, liquid crystal molecule has a higher rotation velocity. The over driving technique uses an over driving voltage that is higher than the target voltage to accelerate the rotation of liquid crystal molecule to reduce the required response time. When the liquid crystal molecule rotates to the angle corresponding to the target voltage, the overdriving voltage is then reduced to the target voltage.
Typically, a look up table as shown in FIG. 1 is used for determining the over driving voltage. According to FIG. 1, two parameters are used to search the over driving voltage. The first parameter is the gray-level value Gn of the present frame. The second parameter is the gray-level value Gn-1 of the previous frame. The Gn′ is the output gray-level value. For example, the gray-level value Gn is 2 and the gray-level value Gn-1 is 253. After searching the look up table, the output gray-level value Gn′ can be obtained from the table as 0.
FIG. 1B is a typical schematic diagram of an over driving circuit. The over driving circuit 100 comprises a buffer 101, a memory 102 and a modifier 103. When the over driving circuit 100 receives an image gray level value, such as the gray level value Gn, of a frame time of a pixel unit, the gray level value Gn is sent to the memory 102 through the buffer 101. Moreover, the gray level value Gn is also sent to the modifier 103. The modifier 103 may search the look up table according to the gray level value Gn and the gray level value Gn-1 of previous frame stored in the memory 102 to determine a corrected gray-level value Gn′. Then, the corrected gray-level value Gn′ is output from a serializer 104.
Then, the corrected gray-level value Gn′ is sent to a corresponding device, such as a driver IC. The driver IC translates the gray-level value Gn′ into corresponding voltage which controls the rotation angle of liquid crystal molecule. Then, the backlight module is turned on to generate light on the liquid crystal molecule. Liquid crystal molecule with specific rotation angle allows the liquid crystal panel to display image with a specific gray level.
The scan lines are scanned from the top to the bottom. Therefore, time difference exists in these scan lines. However, the typical over driving circuit and method only uses two parameters, the gray-level value of the present frame and the gray-level value of the previous frame, to search the corrected gray level value. Such circuit and method does not really consider the time difference among the scan lines.